Taking an active look at a passive problem Continuing complaints from a bus company about repeater malfunctions lead to bringing the repeater into the shop. ‘Drooping’ voltages under various operating conditions eventually lead to the cause.

I operate a small radio and microwave service shop in a medium-size town on California’s Pacific coast. Probably, it is much like your shop. With a staff of three technicians, we have a great time keeping pace with the world and making our customers “happy campers.” I’d like to take you along with me and let you experience some of the fun we have. It was another beautiful day in Paradise. Beautiful, that is, until the phone started ringing. “Radio Shop,” I answered, “this is Wes. How can I help you?” The call was the continuation of a customer’s headache that had been building for a month. The problem was now a migraine. The caller was from the bus company. Trouble again. As I listened, I was about to find out it was “trouble still,” not trouble again. “Ruben!” I hollered, looking for my supervising technician, “Get on the line and listen.” “Right, boss,” Ruben quietly answered from across the shop. The caller was Mark Mason, the dispatcher at the bus company. For more than three weeks, the company had been experiencing trouble with a repeater system. “Wes, the boss is really angry,” said Mark. “We’ve paid you for several service calls to repair this repeater, and it still has problems. It seems like it works OK for a while after each repair, but then it goes nuts.” “What does ‘nuts’ mean, Mark?” I asked. “Well, when some of our mobiles or handies start to talk,” continued Mark, “I hear their clicking to start with, but then silence for about a second or so, then they come on talking normally. Geez, it’s frustrating because I miss the first few words, and then I ask for a repeat, and they miss my asking, and so on. You just have to get it fixed, Wes.” “OK Mark,” I replied, hearing the frustation in his voice, “we’ll get to the bottom of it. Tell your boss that we’re sorry that it wasn’t taken care of sooner. Bye.” “OK, Wes,” ended Mark, “keep us posted. Bye.” “Well, Ruben, I wonder what do we do now. Can you fill me in on what’s going on?” “This was Jerry’s job,” answered Ruben and with another shout, “Jerry!” “Yeah?” Jerry answered meekly from across the shop. “Tell us all about the bus company repeater job,” queried Ruben. “Mark just called the boss, and he’s hot about the fact that it’s been repaired over and over, yet it is still not working correctly. What’s the story?” “Well, I fixed it three times, but it’s still flaky,” claimed Jerry. “I just don’t know where to go with the problem.” “Care to tell us?” I asked. “Yes, tell away,” echoed Ruben. “Well, it started out simple enough as a routine call,” began Jerry. “As you know, the bus company uses a Johnson repeater. First-rate quality. It’s a CR1000 model. It was installed by the other dealer, the old company that was bought out by the national group and then dropped out of servicing.” “That is when we picked the bus company up as a customer,” Ruben remarked. “I remember when they left the area,” I said. “We picked up quite a few customers at that time.” “Well,” continued Jerry, “when I responded to the trouble call, I found the unit would mute on the first second or so of the key-up and then it would un-mute and work OK.” “Isn’t that the repeater that’s been modified?” interjected Ruben, “You know, the one with the Zetron panel and both analog and digital CTCSS?” “Yes,” Jerry confirmed, “it is modified, but it was done right! The installing tech did a good job and documented it clearly and neatly. The modification was to add interconnect, DCSS, CTCSS and all, but very standard and common stuff, you know. And it works well, when it works. But what we have now is a funny problem. I thought I had it solved. I found that when it keys up, the DCSS loses decode lock and then mutes the receiver. I traced this to the receiver detector drifting on key up. The modifications added some capacitors to improve the low-frequency coupling to the tone panel. My first thought was that this was the trouble, so I added some more caps.” “What happened then?” I asked. “That seemed to help. I had Mark and the bus drivers test it, and it worked, so I returned it to service. A week later Mark calls with the same complaint. This time I went further and found that the receiver detector voltages ‘droop’ when the radio keys up. I found this by ‘scoping the power line near the detector. This time I added some extra caps on the power line. The extra caps helped some. I also measured the regulated voltage line in the receiver. It’s a 9V bus regulated through a resistor and a 9.1V zener diode, but it measured only 8V and the voltage bus also ‘drooped’ to about 7V when the transmitter keyed. I replaced the zener to repair the loss of regulation, but the voltage was still low, and still sagged even lower for a moment with transmitter keying. Then I realized the unit might need more, so I talked with Ruben.” “Yeah, I remember,” agreed Ruben. “I remembered the thing had a floating battery on it for power failure protection. I suggested you check it and replace it if questionable, right?” “Right,” connected Jerry as if he were completing Rubens sentence. “Right … and … it helped. I replaced the battery and the problem disappeared. The detector voltages still ‘drooped,’ but not as much, and the detector didn’t drift enough to unlock the decoder and mute the receiver. I had already replaced the regulator, remember. I told Ruben the power should be rewired, because the 12V distribution is ‘daisy-chained’ and probably will never be stable.” “Well, against my best judgment,” I lamented, “you better bring the unit into the shop. You are not going to have a happy customer at this rate, and it’s an hour and half travel each way to the site, so get it in here and we’ll fix it! I’ll have to write off some of the cost to make it up to the customer.” Ruben and Jerry, and Karen, our best technician, had the unit set up and running in a test rack near the bench in no time. They connected it to a dummy load and reprogrammed it to our shop’s own channel so they could test it without messing up anyone else. Then they set up an IFR service monitor to measure the repeater under real use and went to work. “I have a scope on the detector and on the receiver 9V power distribution bus,” indicated Karen. “I want to follow up on what Jerry said the symptoms were. Key it up, Jer!” “You’re on.” announced Jerry, as he used a hand-held shop radio to give a test input to the troublemaker of a repeater. “So are you,” said Karen. “On target, that is. The voltage sags on the 9V line, the detector output drifts, the digital decoder goes out of lock, and the receiver mutes. It is just as you described.” Jerry relished saying, “I told you so. Now what?” “Now, while you get me a cup of coffee,” replied Karen, “I’ll look over the schematic of the receiver. I think I know where you went wrong. You get the coffee, now go on!” “Geez!” whined Jerry, “Did I miss something? OK, OK, I’ll get the coffee.” Karen made a few more checks as she looked over the diagram of the receiver. It was only a few minutes later that she had it all worked out. “Well, Jerry,” she asked, “want to see it?” “How did I miss the the ‘symptom boat’ when everyone was getting aboard?” quizzed Jerry. “You got my attention,” started Karen, “when you mentioned the voltage sag on the zener diode-regulated power distribution bus. Using a zener diode for regulation is a passive design. If the volts are not regulating in a passive design, the load is drawing too much. Your error was to think the power source was the cause of your detector drift. It was a load failure that threw the regulation off. What I found was a bypass capacitor leaking and drawing enough extra current so the 9V bus was too low for the zener to regulate it. Thus, the detector power supply was modulated by the transmitter keying up. It would react the same for any large change in the 12V supply line. If the capacitor had failed completely, the receiver would have quit and would have been easier to troubleshoot. Interestingly, if only tone squelch were used instead of digital squelch, the problem may have continued to exist undetected for some time, and the repeater still would have worked.” “Never,” I started and paused for effect, “let yourself become blinded by the difference between active and passive regulation. When you find a problem in a passive regulator like this one, if anything ‘downstream’ draws excessive current, the regulator is out of business, and the power distribution line also distributes problems throughout the circuit.” “You bet,” added Karen. “My favorite troubleshooting technique for passive regulation failures is to take a current- regulated, adjustable bench power supply, clip it on at the regulator output, apply some volts and use it to ‘force feed’ the bus to locate the fault. Usually this will lead you to the culprit, as it did for me in this Johnson.” “Well, team,” I applauded, “three cheers for Karen, and Jerry, you get to bring the coffee for a whole week. Now, let’s call Mark with the good news and return that radio today.”

The blown fuses and radios are real, but the characters are a composite of many technicians and many shops. Any similarity to persons you know is purely coincidental.